Golf ball

ABSTRACT

Golf ball  2  has a spherical core  4 , a mid layer  6  covering this core  4 , a reinforcing layer  8  covering this mid layer  6 , and a cover  10  covering this reinforcing layer  8 . Base polymer of the mid layer  6  includes a styrene block-containing thermoplastic elastomer and an ionomer resin. The mid layer  6  has a hardness as measured with a Shore D type hardness scale of equal to or less than 54. The mid layer  6  has a thickness of 0.5 mm or greater and 1.6 mm or less. Base polymer of the reinforcing layer  8  is a reaction product between carboxyl group-containing polyurethane and polycarbodiimide. Principal component of the base polymer of the cover  10  is an ionomer resin. The cover  10  has a hardness as measured with a Shore D type hardness scale hardness of equal to or greater than 55. The cover  10  has a thickness of equal to or less than 1.2 mm.

This application claims priority on Patent Application No. 2005-73295filed in JAPAN on Mar. 15, 2005 and Patent Application No. 2005-76551filed in JAPAN on Mar. 17, 2005. The entire contents of these JapanesePatent Applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. More particularly, thepresent invention relates to multi piece golf balls having a core, a midlayer and a cover.

2. Description of the Related Art

Top concern to golf players for golf balls is their flight performances.The golf players particularly place great importance on flight distanceattained upon shots with a driver. Feel at impact is also important togolf balls. Golf players prefer soft feel at impact. Furthermore,durability is also important to golf balls. Golf balls not accompaniedby breakage upon repeated impacts have been desired.

In light of flight performances and feel at impact, golf balls having avariety of structures have been proposed. U.S. Pat. No. 6,106,415 (JP-ANo. H10-179795) discloses a golf ball in which either one of the midlayer or the cover comprises an ionomer resin while the other comprisespolyurethane. U.S. Pat. No. 6,231,460 (JP-A No. H11-253581) discloses agolf ball having a mid layer comprising a thermoplastic polyurethaneelastomer, and a cover comprising an ionomer resin. US2003/083155 (JP-ANo. 2003-52857) discloses a golf ball having a mid layer comprising athermoplastic elastomer, and a cover comprising an ionomer resin. U.S.Pat. No. 6,106,415 (JP-A No. 2001-585) discloses a golf ball having amid layer comprising an urethane resin, and a cover comprising anionomer resin.

Ionomer resins are inferior in compatibility with other resins. Ionomerresins are particularly inferior in compatibility with thermoplasticelastomers. In golf balls having a cover comprising an ionomer resin anda mid layer comprising other resin, adhesion between the mid layer andthe cover may be insufficient. In this type of golf ball, repeatedimpacts may result in detachment of the cover from the mid layer. Thedetachment leads to breakage of the golf ball. In particular, accordingto golf balls having a thin mid layer or a thin cover, breakage is aptto be caused due to insufficient adhesion. An object of the presentinvention is to provide a golf ball that is excellent in the flightperformance, feel at impact and durability.

SUMMARY OF THE INVENTION

A golf ball according to one aspect of the present invention has aspherical core, a mid layer positioned outside of this core, areinforcing layer positioned outside of this mid layer, and a coverpositioned outside of this reinforcing layer. This mid layer has ahardness Hm as measured with a Shore D type hardness scale of equal toor less than 54. Principal component of the base polymer of this coveris an ionomer resin. This cover has a hardness Hc as measured with aShore D type hardness scale of equal to or greater than 55. This coverhas a weight Wc of equal to or less than 5.7 g. This cover has a ratio(Wc/Hc) of the weight Wc to the hardness Hc of equal to or less than0.080.

Preferably, the base polymer of the mid layer comprises 10% by weight ormore and 60% by weight or less styrene block-containing thermoplasticelastomer, and 40% by weight or more and 90% by weight or less ionomerresin.

Preferably, the cover has a thickness Tc of equal to or less than 1.2mm. Preferably, the mid layer has a thickness Tm of 0.5 mm or greaterand 1.6 mm or less.

Preferably, the reinforcing layer comprises a water-based adhesive. Basepolymer of this water-based adhesive is a reaction product betweencarboxyl group-containing polyurethane and polycarbodiimide. Preferably,the reinforcing layer has a thickness of 0.003 mm or greater and 0.050mm or less.

A golf ball according to another aspect of the present invention has aspherical core, a mid layer positioned outside of this core, areinforcing layer positioned outside of this mid layer, and a coverpositioned outside of this reinforcing layer. This mid layer has athickness Tm of 0.5 mm or greater and 1.7 mm or less. Base polymer ofthis cover is an ionomer resin. The cover has a hardness Hc as measuredwith a Shore D type hardness scale of 56 or greater and 65 or less. Thiscover has a thickness Tc of equal to or less than 1.0 mm.

Preferably, the mid layer has a hardness Hm as measured with a Shore Dtype hardness scale of 30 or greater and 54 or less. Preferably, basepolymer of this mid layer comprises 30% by weight or more and 60% byweight or less styrene block-containing thermoplastic elastomer, and 40%by weight or more and 70% by weight or less ionomer resin.

Preferably, base polymer of the reinforcing layer is a reaction productbetween carboxyl group-containing polyurethane and polycarbodiimide.Preferably, the carboxyl group-containing polyurethane and thepolycarbodiimide are water-based.

In this golf ball, firm adhesion between the mid layer and the cover ispermitted via the reinforcing layer. According to this golf ball,sufficient durability is achieved by virtue of the reinforcing layereven though a thin mid layer or a thin cover is employed. The thin midlayer can be responsible for resilience performance of the golf ball.The thin cover can be responsible for feel at impact of the golf ball.Also, the thin cover can optimize launch angle of the golf ball uponimpact. This golf ball is excellent in the flight performance, feel atimpact and durability.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partially cut off cross-sectional view illustrating a golfball according to one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is hereinafter described in detail withappropriate references to the accompanying drawing according to thepreferred embodiments.

A golf ball 2 illustrated in FIG. 1 has a spherical core 4, a mid layer6 covering this core 4, a reinforcing layer 8 covering this mid layer 6,and a cover 10 covering this reinforcing layer 8. Numerous dimples 12are formed on the surface of the cover 10. Of the surface of the cover10, a part except for the dimples 12 is a land 14. This golf ball 2 hasa paint layer and a mark layer to the external side of the cover 10,although these layers are not shown in the FIGURE.

This golf ball 2 has a diameter of from 40 mm to 45 mm. From thestandpoint of conformity to a rule defined by United States GolfAssociation (USGA), the diameter is preferably equal to or greater than42.67 mm. In light of suppression of the air resistance, the diameter ispreferably equal to or less than 44 mm, and more preferably equal to orless than 42.80 mm. Weight of this golf ball 2 is 40 g or greater and 50g or less. In light of attainment of great inertia, the weight ispreferably equal to or greater than 44 g, and more preferably equal toor greater than 45.00 g. From the standpoint of conformity to a ruledefined by USGA, the weight is preferably equal to or less than 45.93 g.

The core 4 is usually obtained through crosslinking of a rubbercomposition. Illustrative examples of preferable base rubber includepolybutadienes, polyisoprenes, styrene-butadiene copolymers,ethylene-propylene-diene copolymers and natural rubbers. In light of theresilience performance, polybutadienes are preferred. When other rubberis used in combination with a polybutadiene, it is preferred thatpolybutadiene is included as a principal component. Specifically, it ispreferred that percentage of polybutadiene occupying the entire baserubber is equal to or greater than 50% by weight, and particularly equalto or greater than 80% by weight. Polybutadienes having a percentage ofcis-1,4 bonds of equal to or greater than 40%, and particularly equal toor greater than 80% are particularly preferred.

For crosslinking of the core 4, a co-crosslinking agent is usually used.Preferable examples of the co-crosslinking agent in light of theresilience performance include monovalent or bivalent metal salts of anα,β-unsaturated carboxylic acid having 2 to 8 carbon atoms. Specificexamples of preferable co-crosslinking agent include zinc acrylate,magnesium acrylate, zinc methacrylate and magnesium methacrylate. Zincacrylate and zinc methacrylate are particularly preferred on the groundsthat a high resilience performance can be achieved.

As a co-crosslinking agent, an α,β-unsaturated carboxylic acid having 2to 8 carbon atoms, and a metal oxide may be also blended. Bothcomponents react in the rubber composition to give a salt. This saltserves is responsible for the crosslinking reaction. Examples ofpreferable α,β-unsaturated carboxylic acid include acrylic acid andmethacrylic acid. Examples of preferable metal oxide include zinc oxideand magnesium oxide.

In light of the resilience performance of the golf ball 2, the amount ofthe co-crosslinking agent to be blended is preferably equal to orgreater than 10 parts by weight, and more preferably equal to or greaterthan 15 parts by weight per 100 parts by weight of the base rubber. Inlight of soft feel at impact, the amount of the co-crosslinking agent tobe blended is preferably equal to or less than 50 parts by weight, andmore preferably equal to or less than 45 parts by weight per 100 partsby weight of the base rubber.

Into the rubber composition for use in the core 4, an organic peroxidemay be preferably blended together with the co-crosslinking agent. Theorganic peroxide serves as a crosslinking initiator. By blending theorganic peroxide, the resilience performance of the golf ball 2 may beimproved. Examples of suitable organic peroxide include dicumylperoxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide.Particularly versatile organic peroxide is dicumyl peroxide.

In light of the resilience performance of the golf ball 2, the amount ofthe organic peroxide to be blended is preferably equal to or greaterthan 0.1 part by weight, more preferably equal to or greater than 0.3part by weight, and particularly preferably equal to or greater than 0.5part by weight per 100 parts by weight of the base rubber. In light ofsoft feel at impact, the amount of the organic peroxide to be blended ispreferably equal to or less than 3.0 parts by weight, and morepreferably equal to or less than 2.5 parts by weight per 100 parts byweight of the base rubber.

Into the core 4 may be blended a filler for the purpose of adjustingspecific gravity and the like. Illustrative examples of suitable fillerinclude zinc oxide, barium sulfate, calcium carbonate and magnesiumcarbonate. Powder of a highly dense metal may be blended as a filler.Specific examples of the highly dense metal include tungsten andmolybdenum. The amount of the filler to be blended is determined adlibitum so that the intended specific gravity of the core 4 can beaccomplished. Particularly preferable filler is zinc oxide. Zinc oxideserves not only to adjust the specific gravity but also as acrosslinking activator. Various kinds of additives such as sulfur, asulfur compound, an anti-aging agent, a coloring agent, a plasticizer, adispersant and the like may be blended at an adequate amount to the core4 as needed. Into the core 4 may be also blended crosslinked rubberpowder or synthetic resin powder.

It is preferred that the core 4 has a diameter of 30.0 mm or greater and41.0 mm or less. Sufficient resilience performance of the golf ball 2 isachieved by setting the diameter to be equal to or greater than 30.0 mm.In this respect, the diameter is more preferably equal to or greaterthan 32.0 mm. Feel at impact by virtue of the mid layer 6 and the flightperformance by virtue of the cover 10 can be achieved by setting thediameter to be equal to or less than 41.0 mm. In this respect, thediameter is more preferably equal to or less than 40.5 mm.

It is preferred that the core 4 has an amount of compressive deformationof 3.0 mm or greater and 6.0 mm or less. The core 4 having an amount ofcompressive deformation of equal to or greater than 3.0 mm isresponsible for soft feel at impact. In this respect, the amount ofcompressive deformation is more preferably equal to or greater than 3.4mm. The core 4 having an amount of compressive deformation of equal toor less than 6.0 mm is responsible for the resilience performance of thegolf ball 2. In this respect, the amount of compressive deformation ismore preferably equal to or less than 5.5 mm.

Upon measurement of the amount of compressive deformation, the core 4 isfirst placed on a hard plate made of metal. Next, a cylinder made ofmetal gradually descends toward the core 4. The core 4 intervenedbetween the bottom face of the cylinder and the hard plate is deformed.A migration distance of the cylinder, starting from the state in whichinitial load of 98 N is applied to the core 4 up to the state in whichfinal load of 1274 N is applied thereto is the amount of compressivedeformation.

Weight of the core 4 is preferably 25 g or greater and 42 g or less.Crosslinking temperature of the core 4 is usually 140° C. or greater and180° C. or less. The crosslinking time period of the core 4 is usually10 minutes or longer and 60 minutes or less. The core 4 may be formedwith two or more layers. Other layer comprising a resin composition or arubber composition may be provided between the core 4 and the mid layer6.

Base polymer of the mid layer 6 comprises a styrene block-containingthermoplastic elastomer. The styrene block-containing thermoplasticelastomer is responsible for the feel at impact of the golf ball 2. Inlight of the feel at impact, it is preferred that proportion of thestyrene block-containing thermoplastic elastomer to total amount of thebase polymer is equal to or greater than 10% by weight, yet equal to orgreater than 25% by weight, still equal to or greater than 30% byweight, and further equal to or greater than 40% by weight.

Examples of the styrene block-containing thermoplastic elastomer includestyrene-butadiene-styrene block copolymers (SBS),styrene-isoprene-styrene block copolymers (SIS),styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenatedSBS, hydrogenated SIS and hydrogenated SIBS. Exemplary hydrogenated SBSmay include styrene-ethylene-butylene-styrene block copolymers (SEBS)Exemplary hydrogenated SIS may includestyrene-ethylene-propylene-styrene block copolymers (SEPS) Exemplaryhydrogenated SIBS may includestyrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In the present invention, exemplary styrene block-containingthermoplastic elastomer may include alloys of olefin and one or two ormore selected from the group consisting of SBS, SIS, SIBS, SEBS, SEPSand SEEPS, and hydrogenated products thereof. Olefin component in thesealloys is presumed to be responsible for the improvement ofcompatibility between the thermoplastic elastomer and the ionomer resin.Use of this alloy may improve the resilience performance of the golfball 2. Preferably, an olefin having 2 to 10 carbon atoms may be used.

In light of the resilience performance of the golf ball 2, content ofthe styrene component in the thermoplastic elastomer is preferably equalto or greater than 10% by weight, more preferably equal to or greaterthan 12% by weight, and particularly preferably equal to or greater than15% by weight. In light of the feel at impact of the golf ball 2, thecontent is preferably equal to or less than 50% by weight, morepreferably equal to or less than 47% by weight, and particularlypreferably equal to or less than 45% by weight.

Specific examples of the styrene block-containing thermoplasticelastomer include “Rabalon® T3339C”, “Rabalon® SJ5400N”, “Rabalon®SJ6400N”, “Rabalon® SJ7400N”, “Rabalon® SJ8400N”, “Rabalon® SJ9400N” and“Rabalon® SR04”, trade names by Mitsubishi Chemical Corporation;“Epofriend® A1010”, a trade name by Daicel Chemical Industries; and“Septon HG-252”, a trade name by Kuraray Co., Ltd.

The base polymer of the mid layer 6 comprises an ionomer resin togetherwith the styrene block-containing thermoplastic elastomer. Ionomerresins are highly elastic. The ionomer resin is responsible for theresilience performance of the golf ball 2. In light of the resilienceperformance, proportion of the ionomer resin to total amount of the basepolymer is preferably equal to or greater than 40% by weight, and morepreferably equal to or greater than 50% by weight.

Examples of preferred ionomer resin include binary copolymers formedwith α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms. Examples of the other ionomer resin include ternarycopolymers formed with α-olefin, an α,β-unsaturated carboxylic acidhaving 3 to 8 carbon atoms and an α,β-unsaturated carboxylate esterhaving 2 to 22 carbon atoms. In the binary copolymer and ternarycopolymer, preferable α-olefin may be ethylene and propylene, whilepreferable α,β-unsaturated carboxylic acid may be acrylic acid andmethacrylic acid. In the binary copolymer and ternary copolymer, a partof the carboxyl group may be neutralized with a metal ion. Illustrativeexamples of the metal ion for use in neutralization include sodium ion,potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion,aluminum ion and neodymium ion. The neutralization may be carried outwith two or more kinds of metal ions. Particularly suitable metal ion inlight of the resilience performance and durability of the golf ball 2 issodium ion, zinc ion, lithium ion and magnesium ion.

Preferable binary copolymer comprises 80% by weight or more and 90% byweight or less α-olefin, and 10% by weight or more and 20% by weight orless α,β-unsaturated carboxylic acid. This binary copolymer providesexcellent resilience performance. Preferable ternary copolymer comprises70% by weight or more and 85% by weight or less α-olefin, 5% by weightor more and 30% by weight or less α,β-unsaturated carboxylic acid, and1% by weight or more and 25% by weight or less α,β-unsaturatedcarboxylate ester. This ternary copolymer provides excellent resilienceperformance.

Particularly preferred ionomer resin is a copolymer formed withethylene, and acrylic acid or methacrylic acid. Preferably, this ionomerresin has a melt index of 3 or greater and 7 or less. This ionomer resinexhibits superior fluidity. Preferably, this ionomer resin has aflexural rigidity of 200 MPa or greater and 400 MPa or less. Thisionomer resin is highly elastic.

Specific examples of the ionomer resin include trade names “Himilan1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”,“Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7318”and “Himilan MK7320”, available from Du Pont-MITSUI POLYCHEMICALS Co.,Ltd.; trade names; “Surlyn® 7930”, “Surlyn® 7940”, “Surlyn® 8140”,“Surlyn® 8940”, “Surlyn® 8945”, “Surlyn® 9120”, “Surlyn® 9910” and“Surlyn® 9945”, available from Dupont; and trade names “IOTEK 7010”,“IOTEK 7030”, “IOTEK 8000” and “IOTEK 8030”, available from EXXONCorporation. Two or more kinds of the ionomer resin may be used incombination. An ionomer resin neutralized with a monovalent metal ion,and an ionomer resin neutralized with a bivalent metal ion may be usedin combination.

Other resin may be also used in the mid layer 6 together with thestyrene block-containing thermoplastic elastomer and the ionomer resin.Illustrative examples of the other resin include thermoplastic polyesterelastomers, thermoplastic polyamide elastomers, thermoplasticpolyurethane elastomers and thermoplastic polyolefin elastomers.

Into the resin composition of the mid layer 6 may be blended a fillerfor the purpose of adjusting specific gravity and the like. Illustrativeexamples of suitable filler include zinc oxide, barium sulfate, calciumcarbonate and magnesium carbonate. Powder of a highly dense metal may bealso blended as a filler. Specific examples of the highly dense metalinclude tungsten and molybdenum. The amount of the filler to be blendedis determined ad libitum so that intended specific gravity of the midlayer 6 can be accomplished. Into the mid layer 6 may be also blended acoloring agent, crosslinked rubber powder or synthetic resin powder.

It is preferred that the mid layer 6 has a hardness Hm of equal to orless than 54. The mid layer 6 having a hardness Hm of equal to or lessthan 54 may be responsible for soft feel at impact. In this respect, thehardness Hm may be equal to or less than 53, more preferably equal to orless than 52, still preferably equal to or less than 51, and furtherpreferably equal to or less than 48. In light of the resilienceperformance of the golf ball 2, the hardness Hm is preferably equal toor greater than 30, more preferably equal to or greater than 33, stillmore preferably equal to or greater than 34, and further preferablyequal to or greater than 37.

In the present invention, the hardness Hm of the mid layer 6 and thehardness Hc of the cover 10 may be measured in accordance with astandard of “ASTM-D 2240-68”. For the measurement, an automated rubberhardness scale (trade name “P1”, available from Koubunshi Keiki Co.,Ltd.) which is equipped with a Shore D type spring hardness scale isused. For the measurement, a sheet which was formed by hot press is usedhaving a thickness of about 2 mm and consisting of the same material asthe mid layer 6 (or the cover 10). Prior to the measurement, the sheetis stored at a temperature of 23° C. for two weeks. When the measurementis carried out, three sheets are overlaid.

The mid layer 6 has a thickness Tm of preferably 0.5 mm or greater and1.6 mm or less. The mid layer 6 having the thickness Tm of equal to orgreater than 0.5 mm may be responsible for soft feel at impact. In thisrespect, the thickness Tm is more preferably equal to or greater than0.6 mm. By setting the thickness Tm to be equal to or less than 1.6 mm,adverse effects of the mid layer 6 on the resilience performance can besuppressed. In this respect, the thickness Tm is more preferably equalto or less than 1.5 mm, and particularly preferably equal to or lessthan 1.2 mm.

The surface of the mid layer 6 may be subjected to a treatment such asbrushing, grinding or the like. This treatment can increase theroughness of the surface. Great roughness improves adhesion between themid layer 6 and the reinforcing layer 8 or the cover 10.

The reinforcing layer 8 lies between the mid layer 6 and the cover 10.The reinforcing layer 8 adheres firmly to the mid layer 6, and adheresfirmly also to the cover 10. As described above, this golf ball 2 has athin mid layer 6. As described later, this golf ball 2 has an extremelythin cover 10. Because the cover 10 adheres firmly to the mid layer 6via the reinforcing layer 8, the mid layer 6 and the cover 10 are noteasily broken irrespective of the mid layer 6 and the cover 10 beingthin. This golf ball 2 is excellent in durability. In other words, thepresence of the reinforcing layer 8 permits to provide the thin midlayer 6 and thin cover 10.

Preferably, base polymer of the reinforcing layer 8 is a reactionproduct between carboxyl group-containing polyurethane andpolycarbodiimide. This reinforcing layer 8 has a three-dimensionalstructure, and is excellent in the strength. Moreover, this reinforcinglayer 8 is excellent in adhesiveness to the mid layer 6 and adhesivenessto the cover 10.

The reinforcing layer 8 preferably comprises a water-based adhesive. Thewater-based adhesive is excellent in workability, and does not adverselyaffect the environment. In particular, it is preferably a reactionproduct between water-based carboxyl group-containing polyurethane andwater-based polycarbodiimide. A liquid obtained by mixing an aqueoussolution or a water dispersion liquid of carboxyl group-containingpolyurethane with an aqueous solution or a water dispersion liquid ofpolycarbodiimide is applied on the mid layer 6 to obtain the reinforcinglayer 8 through a reaction of the carboxyl group-containing polyurethanewith the polycarbodiimide.

Anionic or nonionic carboxyl group-containing polyurethane liquid can beused. In the anionic carboxyl group-containing polyurethane liquid, thepolyurethane is dissolved or dispersed in water through neutralizationof the carboxyl group with a base. In the nonionic carboxylgroup-containing polyurethane liquid, the polyurethane is dispersedforcedly in water with a high shearing force in the presence of anonionic emulsifying agent.

Polyisocyanate component in the carboxyl group-containing polyurethanehas multiple isocyanate groups. Specific examples of the polyisocyanateinclude aromatic polyisocyanates such as 2,4-toluene diisocyanate,2,6-toluene diisocyanate, mixtures of 2,4-toluene diisocyanate and2,6-toluene diisocyanate (TDI), 4,4′-diphenylmethane diisocyanate (MDI),1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate(TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate(TMXDI) and paraphenylene diisocyanate (PPDI) alicyclic polyisocyanatessuch as 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), hydrogenatedxylylene diisocyanate (H₆XDI), hexamethylene diisocyanate (HDI) andisophorone diisocyanate (IPDI); and aliphatic polyisocyanates. Two ormore polyisocyanates may be used in combination. In light of the weatherresistance, TMXDI, XDI, HDI, H₆XDI, IPDI and H₁₂MDI are preferred.

Polyol component in the carboxyl group-containing polyurethane hasmultiple hydroxyl groups. A low molecular weight polyol having amolecular weight of less than 500 and a high molecular weight polyolhaving a molecular weight of equal to or greater than 500 can be used.Examples of the low molecular weight polyol include diol and triol.Specific examples of the diol include ethylene glycol, diethyleneglycol, triethyleneglycol, 1,3-butanediol, 1,4-butanediol, neopentylglycol and 1,6-hexanediol. Specific examples of the triol includeglycerin, trimethylolpropane and hexanetriol. Examples of the highmolecular weight polyol include polyether polyols such aspolyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG) andpolyoxytetramethylene glycol (PTMG); condensed polyester polyols such aspolyethylene adipate (PEA), polybutylene adipate (PBA) andpolyhexamethylene adipate (PHMA); lactone based polyester polyols suchas poly-ε-caprolactone (PCL); polycarbonate polyols such aspolyhexamethylene carbonate; and acrylic polyols. Two or more kinds ofpolyols may be used in combination. Polyols having a weight averagemolecular weight of 50 or greater and 2000 or less, particularly 100 orgreater and 1000 or less are particularly preferred.

The carboxyl group-containing polyurethane may contain a polyaminecomponent. Examples of the polyamine component include aliphaticpolyamines such as ethylenediamine, propylenediamine andhexamethylenediamine; aromatic polyamines such as tolylenediamine,xylylenediamine and diaminodiphenylmethane; alicyclic polyamines such asdiaminocyclohexylmethane, piperazine and isophoronediamine; andhydrazines such as hydrazine, succinic acid dihydrazide, adipic aciddihydrazide and phthalic acid dihydrazide. Alkanolamines such asdiethanolamine and monoethanolamine may be used.

The water-based polycarbodiimide has two or more carbodiimide groupswithin the molecule, and is thus soluble or dispersible in water.Suitably, a polycarbodiimide resin having a structure represented by thefollowing formula (I) may be used.S1-K—(R—N═C═N)_(n)—R—K—S2  (I)

In the above formula (I), S1 and S2 represent a hydrophilic segment; Rrepresents a residue yielded by excluding the isocyanate group fromdiisocyanate; K represents a bond formed by a reaction of the isocyanategroup with the hydrophilic segment; and n represents an average degreeof polymerization. The symbol n represents an integer of 2 or greaterand 100 or less. Illustrative examples of the hydrophilic segmentinclude nonionic segments having an ethyleneoxide chain; anionicsegments having a sulfonate salt, a sulfate salt or the like; andcationic segments having a quaternary ammonium salt. Specific examplesof the polycarbodiimide resin represented by the above formula (I)include tetramethylxylylenecarbodiimide anddicyclohexylmethanecarbodiimide.

Carbodiimide equivalent in the water-based polycarbodiimide ispreferably 100 or greater and 500 or less. By setting the carbodiimideequivalent to be equal to or greater than 100, the reinforcing layer 8having great crosslinking density is obtained. This reinforcing layer 8is responsible for adhesion between the mid layer 6 and the cover 10. Inthis respect, the carbodiimide equivalent is more preferably equal to orgreater than 150, and particularly preferably equal to or greater than250. By using the polycarbodiimide having the carbodiimide equivalent ofequal to or less than 500, the reaction with the carboxylgroup-containing polyurethane can be completed in a short period oftime. In this respect, the carbodiimide equivalent is more preferablyequal to or less than 450, and particularly preferably equal to or lessthan 400. The carbodiimide equivalent refers to the chemical formulaweight of polycarbodiimide per mol of the carbodiimide group.

Solid content of the polycarbodiimide in the aqueous solution or thewater dispersion liquid is preferably equal to or greater than 20% byweight, and more preferably equal to or greater than 30% by weight. Thesolid content is preferably equal to or less than 80% by weight, andmore preferably equal to or less than 70% by weight. Illustrativeexamples of preferable aqueous solution of polycarbodiimide includetrade names of “Carbodilite® E-01”, “Carbodilite® E-02” and“Carbodilite® E-03A”, available from Nisshin Spinning Co., Ltd.

Amount of the polycarbodiimide to be blended is preferably 5 parts byweight or greater and 25 parts by weight or less per 100 parts by weightof the carboxyl group-containing polyurethane. By setting the amount tobe equal to or greater than 5 parts by weight, a reinforcing layer 8having great crosslinking density may be obtained. This reinforcinglayer 8 is responsible for adhesion between the mid layer 6 and thecover 10. In this respect, the amount is more preferably equal to orgreater than 6 parts by weight, and particularly preferably equal to orgreater than 7 parts by weight. By setting the amount to be equal to orless than 25 parts by weight, the reaction with the carboxylgroup-containing polyurethane can be completed in a short period oftime. In this respect, the amount is more preferably equal to or lessthan 23 parts by weight, and particularly preferably equal to or lessthan 20 parts by weight.

The reinforcing layer 8 may comprise an oil-based adhesive. As the basepolymer of this reinforcing layer 8, a two-component cured thermosettingresin may be typically used. Specific examples of the two-componentcured thermosetting resin include epoxy resins, urethane resins, acrylicresins, polyester based resins and cellulose based resins. In light ofthe mechanical characteristics (e.g., strength at break) and durabilityof the reinforcing layer 8, two-component cured epoxy resins andtwo-component cured urethane resins are preferred.

The two-component cured epoxy resin is obtained by curing an epoxy resinwith a polyamide based curing agent. Illustrative examples of the epoxyresin for use in the two-component cured epoxy resin include bisphenol Atype epoxy resins, bisphenol F type epoxy resins and bisphenol AD typeepoxy resins. The bisphenol A type epoxy resin is obtained by a reactionof bisphenol A with an epoxy group-containing compound such asepichlorohydrin. The bisphenol F type epoxy resin is obtained by areaction of bisphenol F with an epoxy group-containing compound. Thebisphenol AD type epoxy resin is obtained by a reaction of bisphenol ADwith an epoxy group-containing compound. In light of the balance amongsoftness, chemical resistance, heat resistance and toughness, thebisphenol A type epoxy resins are preferred.

The polyamide based curing agent has multiple amino groups and one ormore amide groups. This amino group can react with an epoxy group.Specific examples of the polyamide based curing agent include polyamideamine curing agents and modified products of the same. The polyamideamine curing agent is obtained by a condensation reaction of apolymerized fatty acid with a polyamine. Typical polymerized fatty acidmay be obtained by heating naturally occurring fatty acids containinglarge amounts of unsaturated fatty acids such as linoleic acid,linolenic acid and the like in the presence of a catalyst to perfect thesynthesis. Specific examples of the unsaturated fatty acid include talloil, soybean oil, linseed oil and fish oil. Polymerized fatty acidshaving a dimer content of equal to or greater than 90% by weight and atrimer content of equal to or less than 10% by weight, and beinghydrogenated are preferred. Illustrative examples of preferred polyamineinclude polyethylenediamine, polyoxyalkylenediamine and derivativesthereof.

Upon mixing of the epoxy resin and the polyamide based curing agent,ratio of epoxy equivalent of the epoxy resin and amine active hydrogenequivalent of the polyamide based curing agent is preferably 1.0/1.4 orgreater and 1.0/1.0 or less.

The two-component cured urethane resin is obtained by a reaction of abase material and a curing agent. A two-component cured urethane resinobtained by a reaction of a base material containing a polyol componentwith a curing agent containing polyisocyanate or a derivative thereof,or a two-component cured urethane resin obtained by a reaction of a basematerial containing isocyanate group-ended urethane prepolymer with acuring agent having an active hydrogen may be used. In particular,two-component cured urethane resins obtained by a reaction of a basematerial containing a polyol component with a curing agent containingpolyisocyanate or a derivative thereof are preferred.

It is preferred that an urethane polyol is used as the polyol componentof the base material. The urethane polyol has urethane bonds and two ormore hydroxyl groups. Preferably, the urethane polyol has a hydroxylgroup at its end. The urethane polyol may be obtained by allowing apolyol and a polyisocyanate to react at a ratio such that an excessivemolar ratio of the hydroxyl group of the polyol component to theisocyanate group of polyisocyanate is attained.

The polyol for use in production of the urethane polyol has multiplehydroxyl groups. Polyol having a weight average molecular weight of 50or greater and 2000 or less, and particularly 100 or greater and 1000 orless is preferred. Examples of the polyol having a low molecular weightinclude diol and triol. Specific examples of the diol include ethyleneglycol, diethylene glycol, triethylene glycol, 1,3-butanediol,1,4-butanediol, neopentyl glycol and 1,6-hexanediol. Specific examplesof the triol include trimethylolpropane and hexanetriol. Examples of thepolyol having a high molecular weight include polyether polyols such aspolyoxyethylene glycol (PEG), polyoxypropylene glycol (PPG) andpolyoxytetramethylene glycol (PTMG); condensed polyester polyols such aspolyethylene adipate (PEA), polybutylene adipate (PBA) andpolyhexamethylene adipate (PHMA); lactone based polyester polyols suchas poly-ε-caprolactone (PCL); polycarbonate polyols such aspolyhexamethylene carbonate; and acrylic polyols. Two or more kinds ofpolyols may be used in combination.

Polyisocyanate for use in production of the urethane polyol has multipleisocyanate groups. Specific examples of the polyisocyanate includearomatic polyisocyanates such as 2,4-toluene diisocyanate, 2,6-toluenediisocyanate, mixtures of 2,4-toluene diisocyanate and 2,6-toluenediisocyanate (TDI), 4,4′-diphenylmethanediisocyanate (MDI)1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate(TODI), xylylene diisocyanate (XDI), tetramethylxylylene diisocyanate(TMXDI) and paraphenylene diisocyanate (PPDI); alicyclic polyisocyanatessuch as 4,4′-dicyclohexylmethane diisocyanate (H₁₂MDI), hydrogenatedxylylene diisocyanate (H₆XDI), hexamethylene diisocyanate (HDI) andisophorone diisocyanate (IPDI); and aliphatic polyisocyanates. Two ormore polyisocyanates may be used in combination. In light of the weatherresistance, TMXDI, XDI, HDI, H₆XDI, IPDI and H₁₂MDI are preferred.

In the reaction of the polyol with the polyisocyanate for producing theurethane polyol, any known catalyst may be used. Typical catalyst may bedibutyltin dilaurate.

In light of strength of the reinforcing layer 8, proportion of urethanebonds included in the urethane polyol is preferably equal to or greaterthan 0.1 mmol/g. In light of the following capability of the reinforcinglayer 8 to the cover 10, the proportion of urethane bonds included inthe urethane polyol is preferably equal to or less than 5 mmol/g. Theproportion of urethane bonds may be adjusted by regulating the molecularweight of the polyol to be a raw material, and by regulating compoundingratio of the polyol and the polyisocyanate.

In light of a short time period required for the reaction of the basematerial with the curing agent, the urethane polyol has a weight averagemolecular weight of preferably equal to or greater than 4000, and morepreferably equal to or greater than 4500. In light of adhesiveness ofthe reinforcing layer 8, the urethane polyol has a weight averagemolecular weight of preferably equal to or less than 10000, and morepreferably equal to or less than 9000.

In light of adhesiveness of the reinforcing layer 8, the urethane polyolhas a hydroxyl value (mgKOH/g) of preferably equal to or greater than15, and more preferably equal to or greater than 73. In light of a shorttime period required for the reaction of the base material with thecuring agent, the urethane polyol has a hydroxyl value of preferablyequal to or less than 130, and more preferably equal to or less than120.

The base material may contain, in addition to the urethane polyol, apolyol not having any urethane bond. The aforementioned polyol that maybe a raw material of the urethane polyol can be used in the basematerial. Polyols that are compatible with the urethane polyol arepreferred. In light of a short time period required for the reaction ofthe base material with the curing agent, proportion of the urethanepolyol in the base material is preferably equal to or greater than 50%by weight, and more preferably equal to or greater than 80% by weightbased on the solid content. Ideally, this proportion is 100% by weight.

The curing agent contains polyisocyanate or a derivative thereof. Theaforementioned polyisocyanate that is a raw material of the urethanepolyol may be used as the curing agent.

The reinforcing layer 8 comprises the two-component cured thermosettingresin is obtained by applying a liquid prepared by dissolving ordispersing the base material and the curing agent in a solvent on thesurface of the mid layer 6. In light of the workability, applicationwith a spray gun is preferred. Following the application, the solvent isvolatilized to permit a reaction of the base material with the curingagent thereby forming the reinforcing layer 8. Illustrative examples ofpreferred solvent include toluene, isopropyl alcohol, xylene, methylethyl ketone, methyl isobutyl ketone, ethylene glycol monomethyl ether,ethylbenzene, propylene glycol monomethyl ether, isobutyl alcohol andethyl acetate.

In light of the durability of the golf ball 2, the reinforcing layer 8has a thickness of preferably equal to or greater than 0.003 mm, andmore preferably equal to or greater than 0.005 mm. In light of easyformation of the reinforcing layer 8, it is preferred that the thicknessis equal to or less than 0.30 mm, still more, equal to or less than 0.10mm, yet more, equal to or less than 0.05 mm, and furthermore, equal toor less than 0.02 mm. The thickness is measured by observation of thecross section of the golf ball 2 with a micro scope. When the surface ofthe mid layer 6 has roughness resulting from the surface rougheningtreatment, the thickness is measured immediately above the protrudedportion.

Principal component of the base polymer of the cover 10 is an ionomerresin. The aforementioned binary copolymer ionomer resin and the ternarycopolymer ionomer resin can be used in the cover 10. The ionomer resinis highly elastic. By using the ionomer resin in the cover 10, a golfball 2 that is excellent in resilience performance is obtained.

As described above, the mid layer 6 comprises a styrene block-containingthermoplastic elastomer. When this mid layer 6 and the cover 10comprising the ionomer resin are directly attached, adhesivenesstherebetween may become insufficient. In the present golf ball 2, thereinforcing layer 8 lies between the mid layer 6 and the cover 10,therefore, firm adhesion between the mid layer 6 and the cover 10 ispermitted via this reinforcing layer 8.

Other resin may be used in the cover 10 together with the ionomer resin.Examples of the other resin include styrene block-containingthermoplastic elastomers, thermoplastic polyurethane elastomers,thermoplastic polyester elastomers, thermoplastic polyamide elastomersand thermoplastic polyolefin elastomers. When the other resin is used incombination with the ionomer resin, proportion of the ionomer resin intotal amount of the base polymer is preferably equal to or greater than50% by weight, and more preferably equal to or greater than 70% byweight, and particularly preferably equal to or greater than 90% byweight.

Into the cover 10 may be blended a coloring agent such as titaniumdioxide, a filler such as barium sulfate, a dispersant, an antioxidant,an ultraviolet absorbent, a light stabilizer, a fluorescent agent, afluorescent brightening agent and the like in an appropriate amount asneeded. Powder of a highly dense metal such as tungsten, molybdenum orthe like may be also blended into the cover 10 for the purpose ofadjusting the specific gravity.

The cover 10 has a hardness Hc of equal to or greater than 55. The cover10 having a hardness Hc of equal to or greater than 55 is responsiblefor the resilience performance. In this respect, the hardness Hc ispreferably equal to or greater than 56, more preferably equal to orgreater than 57, and still more preferably equal to or greater than 58.In light of soft feel at impact, the hardness Hc is preferably equal toor less than 65, more preferably equal to or less than 64, and stillmore preferably equal to or less than 63.

The cover 10 has a thickness Tc of equal to or less than 1.2 mm. Thiscover 10 is extremely thin. By employing such a thin cover 10, a greatlaunch angle can be accomplished upon impact with a golf club. The greatlaunch angle is responsible for great flight distance. Moreover, byemploying a thin cover 10, adverse effects of the cover 10 on the feelat impact can be suppressed. In this respect, the thickness Tc ispreferably equal to or less than 1.0 mm, more preferably equal to orless than 0.9 mm, still more preferably equal to or less than 0.8 mm,and even more preferably equal to or less than 0.5 mm. Although the thincover 10 is apt to be broken upon repeated impacts, the reinforcinglayer 8 suppresses the breakage in this golf ball 2. This golf ball 2 isexcellent in the flight performance, feel at impact and durability. Inlight of ease of production, the cover 10 has a thickness Tc of equal toor greater than 0.1 mm, and more preferably equal to or greater than 0.3mm. The thickness Tc is measured immediately below the land 14 where nodimple 12 exists.

The cover 10 has a weight Wc of equal to or less than 5.7 g. By settingthe weight Wc to be equal to or less than 5.7 g, a great launch anglecan be provided. The great launch angle is responsible for great flightdistance. Furthermore, by setting the weight Wc to be equal to or lessthan 5.7 g, adverse effect on the feel at impact exerted by the cover 10may be suppressed. In this respect, the weight Wc is more preferablyequal to or less than 5.3 g, and particularly preferably equal to orless than 4.8 g. In light of ease of production, the cover 10 has aweight Wc of preferably equal to or greater than 1.5 g, more preferablyequal to or greater than 1.7 g, and particularly preferably equal to orgreater than 1.9 g.

Ratio (Wc/Hc) of the weight Wc to the hardness Hc in the cover 10 isequal to or less than 0.080. By setting the ratio (Wc/Hc) to be equal toor less than 0.080, great launch angle and great resilience coefficientcan be both achieved simultaneously. In this respect, the ration (Wc/Hc)is more preferably equal to or less than 0.078, and particularlypreferably equal to or less than 0.075. The ratio (Wc/Hc) is preferablyequal to or greater than 0.030. By setting the ratio (Wc/Hc) to be equalto or greater than 0.030, productivity and feel at impact can be bothachieved simultaneously. The ratio (Wc/Hc) is an index that representsthe degree of contribution of the cover 10 to the launch angle,resilience coefficient, productivity and feel at impact of the golf ball2.

Amount of compressive deformation of the golf ball 2 is preferably 2.0mm or greater and 4.5 mm or less. The golf ball 2 having the amount ofcompressive deformation of equal to or greater than 2.0 mm is excellentin the feel at impact. In this respect, the amount of compressivedeformation is more preferably equal to or greater than 2.2 mm, andparticularly preferably equal to or greater than 2.3 mm. The golf ball 2having the amount of compressive deformation of equal to or less than4.5 mm is excellent in the resilience performance. In this respect, theamount of compressive deformation is more preferably equal to or lessthan 4.3 mm, and particularly preferably equal to or less than 4.0 mm.The amount of compressive deformation of the golf ball 2 is measured inaccordance with the method for measuring the amount of compressivedeformation of the core 4 as described above.

EXAMPLES Experiment 1 Example 1

A type x rubber composition was obtained by kneading 100 parts by weightof high cis-polybutadiene (tradename “BR-730”, available from JSRCorporation), 25 parts by weight of zinc acrylate, 10 parts of zincoxide, an adequate amount of barium sulfate, 0.5 part by weight ofdiphenyl disulfide (manufactured by Sumitomo Seika Chemicals Co., Ltd.)and 0.8 part by weight of dicumyl peroxide (manufactured by NOFCorporation). This rubber composition was placed into a mold havingupper and lower mold half each having a hemispherical cavity, and heatedat 170° C. for 20 minutes to obtain a core having a diameter of 39.2 mm.

In a biaxial extruder having a screw diameter of 45 mm, which provides arotational velocity of the screw being 200 rpm, and having L/D of 35, atype b resin composition shown in Table 1 below was obtained.Temperature in the die during extrusion of the resin composition wasfrom 180° C. to 230° C. The core was covered by this resin compositiontherearound with injection molding to obtain a mid layer. This mid layerhad a thickness Tm of 1.0 mm, and a hardness Hm of 42.

An aqueous solution of carboxyl group-containing polyurethane (tradename “W-615”, available from Mitsui Takeda Chemicals, Inc.) in an amountof 100 parts by weight was mixed with 10 parts by weight of a waterdispersion liquid of polycarbodiimide (trade name “Carbodilite® E-03A”,supra) This aqueous solution of carboxyl group-containing polyurethanehad an acid value of 9 mg KOH/g, and a solid content of 35% by weight.This water dispersion liquid of polycarbodiimide had a carbodiimideequivalent of 365, and a solid content of 40% by weight. This mixturewas applied on the mid layer by a spray gun to obtain a reinforcinglayer having a thickness of 0.008 mm.

A type e resin composition shown in Table 2 below was obtained in theaforementioned biaxial extruder. Half shell was obtained from this resincomposition with compression molding. The spherical body composed of thecore, the mid layer and the reinforcing layer was covered by two piecesof the half shell, and placed into a mold having upper and lower moldhalf each having a hemispherical cavity to obtain a cover by compressionmolding. The cover had a thickness Tc of 0.8 mm. A paint layer wasformed around this cover to give a golf ball of Example 1. This golfball had a diameter of 42.8 mm.

Examples 3 to 4

In a similar manner to Example 1 except that the diameter of the coreand the thickness of the mid layer were as listed in Table 3 below, golfballs of Examples 3 to 4 were obtained.

Examples 2, 5 and 6, and Comparative Example 1

In a similar manner to Example 1 except that the type of the resincomposition of the mid layer was as listed in Table 3 below, golf ballsof Examples 2, 5 and 6, and Comparative Example 1 were obtained. Detailsof the resin composition are shown in Table 2 below.

Comparative Example 2

In a similar manner to Example 1 except that the type of the resincomposition of the cover was as listed in Table 4 below, golf ball ofComparative Example 2 was obtained. Details of the resin composition areshown in Table 2 below.

Examples 7 to 8 and Comparative Example 3

In a similar manner to Example 1 except that the diameter of the coreand the thickness of the cover were as listed in Table 4 below, golfballs of Examples 7 to 8 were obtained. In a similar manner to Example 1except that the type of the rubber composition and the diameter of thecore, and the thickness of the cover were as listed in Table 4 below,golf ball of Comparative Example 3 was obtained. Details of the rubbercomposition are shown in Table 1 below.

Example 9

In a similar manner to Example 1 except that a coating compositioncomprising a two-component cured epoxy resin as the base polymer (tradename “POLIN 750LE”, available from Shinto Paint Co., Ltd.) was used forforming the reinforcing layer, golf ball of Example 9 was obtained. Thiscoating composition was prepared by mixing 100 parts by weight of thebase material with 100 parts by weight of the curing agent. This basematerial contains a 30% by weight bisphenol A type epoxy resin and a 70%by weight solvent. This curing agent contains 40% by weight modifiedpolyamide amine, 5% by weight titanium dioxide, and a 55% by weightsolvent.

Comparative Example 4

In a similar manner to Example 1 except that the reinforcing layer wasnot provided, golf ball of Comparative Example 4 was obtained.

Examples 10 to 11

In a similar manner to Example 1 except that specifications of the core,the mid layer and the cover were as listed in Table 4 below, golf ballsof Examples 10 to 11 were obtained.

[Measurement of Amount of Compressive Deformation]

In a similar manner to the measurement of the amount of compressivedeformation of the core described above, amount of compressivedeformation of the golf ball was measured. The results are shown inTable 3 and Table 4 below.

[Evaluation of Durability]

A driver with a metal head was attached to a swing machine availablefrom True Temper Co. Then the machine condition was set to give the headspeed of 45 m/sec, and the golf balls were hit therewith to allow forimpact on a plate for impact. Accordingly, the number of times of thehitting until the golf ball was broken was counted. Mean values of themeasurement of six golf balls were determined. The results are shown inTable 3 and Table 4 below in terms of indices on the basis of the valuein Comparative Example 4 being presumed as 100.

[Measurement of Flight Distance]

A driver with a metal head was attached to the swing machine describedabove. Then the machine condition was set to give the head speed of 45m/sec, and the golf balls were hit therewith. Accordingly, the distancefrom the launching point to the point where the ball stopped wasmeasured. Mean values of 10 times measurement are shown in Table 3 andTable 4 below.

[Evaluation of Feel at Impact]

Using a driver, the golf balls were hit by a golf player, and the feelat impact was rated among the following four ranks of from “A” to “D”.

A: extremely favorable

B: favorable

C: somewhat unfavorable

D: unfavorable.

The results are presented in Table 3 and Table 4 below. TABLE 1 Rubbercomposition of core (part by weight) Type x y BR-730 100 100 Zincacrylate 25 24 Zinc oxide 10 10 Barium sulfate adequate amount adequateamount Diphenyl disulfide 0.5 0.5 Dicumyl peroxide 0.8 0.8

TABLE 2 Resin compositions of mid layer and cover (part by weight) Typea b c d e f g Surlyn 8945 20 26 37 40 45 50 — Surlyn 9945 20 26 37 40 4550 — Rabalon SR04 60 48 26 20 10 — — Elastollan XNY97A * — — — — — — 100Titanium dioxide 4.0 4.0 4.0 4.0 4.0 4.0 4.0 Ultra Marine Blue 0.1 0.10.1 0.1 0.1 0.1 0.1 Hardness (Shore D) 30 42 53 56 60 63 42* Thermoplastic polyurethane elastomer, available from BASF Co., Ltd.

TABLE 3 Results of evaluation Compara. Example Example Example ExampleExample Example Example 2 3 1 4 5 6 1 Core Type x x x x x x x Diameter(mm) 39.2 39.8 39.2 38.8 39.2 39.2 39.2 Mid Type a b b b g c d layerHardness Hm 30 42 42 42 42 53 56 (Shore D) Thickness Tm (mm) 1.0 0.7 1.01.2 1.0 1.0 1.0 Reinforcing Base Polymer water- water- water- water-water- water- water- layer based based based based based based basedurethane urethane urethane urethane urethane urethane urethane CoverType e e e e e e e Hardness Hc 60 60 60 60 60 60 60 (Shore D) ThicknessTc (mm) 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Weight Wc (g) 4.0 4.0 4.0 4.0 4.04.0 4.0 Wc/Hc 0.067 0.067 0.067 0.067 0.067 0.067 0.067 Amount ofcompressive 3.6 3.4 3.5 3.6 3.5 3.5 3.3 deformation (mm) of ballDurability 140 143 143 144 134 144 144 Travel distance (m) 252 259 257256 252 256 255 Feel at impact A A A A B B C

TABLE 4 Results of evaluation Compara. Compara. Compara. Example ExampleExample Example Example Example Example Example 2 7 8 3 9 4 10 11 CoreType x x x y x x x x Diameter (mm) 39.2 39.8 38.8 38.0 39.2 39.2 38.840.4 Mid Type b b b b b b b b layer Hardness Hm 42 42 42 42 42 42 42 42(Shore D) Thickness Tm (mm) 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.7 ReinforcingBase polymer water- water- water- water- epoxy — water- water- layerbased based based based based based urethane urethane urethane urethaneurethane urethane Cover Type c e e e e e f e Hardness Hc 53 60 60 60 6060 63 60 (Shore D) Thickness Tc (mm) 0.8 0.5 1.0 1.4 0.8 0.8 1.0 0.5Weight Wc (g) 4.0 2.5 4.7 6.3 4.0 4.0 4.7 2.5 Wc/Hc 0.075 0.042 0.0750.105 0.067 0.067 0.075 0.042 Amount of compressive 3.6 3.6 3.5 3.4 3.53.5 3.4 3.7 deformation (mm) of ball Durability 145 142 142 142 142 100142 130 Travel distance (m) 240 259 256 242 257 248 256 260 Feel atimpact B A A B A A A A

As is clear from Table 3 and Table 4, the golf balls of Examples areexcellent in all terms of the durability, flight performance and feel atimpact. Therefore, advantages of the present invention are clearlysuggested by these results of evaluation.

Experiment 2 Example 12

A type x rubber composition shown in Table 1 above was obtained bykneading 100 parts by weight of high cis-polybutadiene (trade name“BR-730”, available from JSR Corporation), 25 parts by weight of zincacrylate, 10 parts of zinc oxide, an adequate amount of barium sulfate,0.5 part by weight of diphenyl disulfide and 0.8 part by weight ofdicumyl peroxide. This rubber composition was placed into a mold havingupper and lower mold half each having a hemispherical cavity, and heatedat 170° C. for 20 minutes to obtain a core having a diameter of 38.8 mm.

In a biaxial extruder having a screw diameter of 45 mm, which provides arotational velocity of the screw being 200 rpm, and having L/D of 35, atype B resin composition shown in Table 5 below was obtained.Temperature in the die during extrusion of the resin composition wasfrom 180° C. to 230° C. The core was covered by this resin compositiontherearound with injection molding to obtain a mid layer. This mid layerhad a thickness Tm of 1.2 mm, and a hardness Hm of 42.

An aqueous solution of carboxyl group-containing polyurethane (tradename “W-615”, available from Mitsui Takeda Chemicals, Inc.) in an amountof 100 parts by weight was mixed with 10 parts by weight of a waterdispersion liquid of polycarbodiimide (trade name “Carbodilite® E-03A”,supra) This aqueous solution of carboxyl group-containing polyurethanehad an acid value of 9 mg KOH/g, and a solid content of 35% by weight.This water dispersion liquid of polycarbodiimide had a carbodiimideequivalent of 365, and a solid content of 40% by weight. This mixturewas applied on the mid layer by a spray gun to obtain a reinforcinglayer having a thickness of 0.008 mm.

A type F resin composition shown in Table 5 below was obtained in theaforementioned biaxial extruder. Half shell was obtained from this resincomposition with compression molding. The spherical body composed of thecore, the mid layer and the reinforcing layer was covered by two piecesof the half shell, and placed into a mold having upper and lower moldhalf each having a hemispherical cavity to obtain a cover by compressionmolding. The cover had a thickness Tc of 0.8 mm. A paint layer wasformed around this cover to give a golf ball of Example 12. This golfball had a diameter of 42.8 mm.

Examples 14 to 15 and Comparative Example 5

In a similar manner to Example 12 except that the diameter of the coreand the thickness of the cover were as listed in Table 6 below, golfballs of Examples 14 to 15 and Comparative Example 5 were obtained.

Example 13 and Comparative Example 6

In a similar manner to Example 12 except that the diameter of the coreand the thickness of the mid layer were as listed in Table 6 below, golfball of Example 13 was obtained. In a similar manner to Example 12except that a type y rubber composition was used for the core, and thatthe diameter of the core and the thickness of the mid layer were aslisted in Table 6 below, golf ball of Comparative Example 6 wasobtained. Details of the type y rubber composition are shown in Table 1above.

Comparative Example 7

In a similar manner to Example 1 except that the type of the resincomposition of the cover was as listed in Table 7 below, golf ball ofComparative Example 7 was obtained. Details of the resin composition areshown in Table 5 below.

Examples 16 to 18

In a similar manner to Example 12 except that the type of the resincomposition of the mid layer was as listed in Table 7 below, golf ballsof Examples 16 to 18 were obtained. Details of the resin composition areshown in Table 5 below.

Example 19

In a similar manner to Example 1 except that a coating compositioncomprising a two-component cured epoxy resin as the base polymer (tradename “POLIN 750LE”, available from Shinto Paint Co., Ltd.) was used forforming the reinforcing layer, golf ball of Example 19 was obtained.This coating composition was prepared by mixing 100 parts by weight ofthe base material with 100 parts by weight of the curing agent. Thisbase material contains a 30% by weight bisphenol A type epoxy resin anda 70% by weight solvent. This curing agent contains 40% by weightmodified polyamide amine, 5% by weight titanium dioxide, and a 55% byweight solvent.

Comparative Example 8

In a similar manner to Example 12 except that the reinforcing layer wasnot provided, golf ball of Comparative Example 16 was obtained.

[Evaluation]

Similarly to the aforementioned Experiment 1, the amount of compressivedeformation, the durability, the flight distance and the feel at impactof the golf balls were evaluated. The results are presented in Table 6and Table 7 below. In connection with the durability, indices on thebasis of the value in Comparative Example 8 being presumed as 100 arepresented in Table 6 and Table 7 below. TABLE 5 Specifications of midlayer and cover (part by weight) Type A B C D E F Surlyn 8945 18 26 3335 40 45 Surlyn 9945 18 26 33 35 40 45 Rabalon SR04 64 48 34 30 20 10Titanium dioxide 4.0 4.0 4.0 4.0 4.0 4.0 Ultra Marine Blue 0.1 0.1 0.10.1 0.1 0.1 Hardness (Shore D) 28 42 48 50 56 60

TABLE 6 Results of evaluation Compara. Compara. Example Example ExampleExample Example Example 13 14 12 15 5 6 Core Type x x x x x y Diameter(mm) 39.8 39.4 38.8 38.4 38.0 37.4 Mid Type B B B B B B layer Hardness(Shore D) 42 42 42 42 42 42 Thickness (mm) 0.7 1.2 1.2 1.2 1.2 1.9Reinforcing Base Polymer water- water- water- water- water- water- layerbased based based based based based urethane urethane urethane urethaneurethane urethane Cover Type F F F F F F Hardness (Shore D) 60 60 60 6060 60 Thickness (mm) 0.8 0.5 0.8 1.0 1.2 0.8 Amount of compressive 3.33.4 3.4 3.4 3.3 3.3 deformation (mm) of ball Durability 133 135 138 140139 138 Travel distance (m) 257 258 256 253 246 245 Feel at impact A A AA C B

TABLE 7 Results of evaluation Compara. Compara. Example Example ExampleExample Example Example 7 16 17 18 19 8 Core Type x x x x x x Diameter(mm) 38.8 38.8 38.8 38.8 38.8 38.8 Mid Type B A C E B B layer Hardness(Shore D) 42 28 48 56 42 42 Thickness (mm) 1.2 1.2 1.2 1.2 1.2 1.2Reinforcing Base Polymer water- water- water- water- epoxy — layer basedbased based based urethane urethane urethane urethane Cover Type D F F FF F Hardness (Shore D) 50 60 60 60 60 60 Thickness (mm) 0.8 0.8 0.8 0.80.8 0.8 Amount of compressive 3.5 3.6 3.3 3.2 3.4 3.4 deformation (mm)of ball Durability 140 133 140 143 132 100 Travel distance (m) 240 249254 253 254 247 Feel at impact B B B B A A

As is clear from Table 6 and Table 7, the golf balls of Examples areexcellent in all terms of the durability, flight performance and feel atimpact. Therefore, advantages of the present invention are clearlysuggested by these results of evaluation.

The foregoing description is just for illustrative examples, therefore,various modifications can be made in the scope without departing fromthe principles of the present invention.

1. A golf ball which comprises a spherical core, a mid layer positionedoutside of the core, a reinforcing layer positioned outside of the midlayer, and a cover positioned outside of the reinforcing layer, said midlayer having a hardness Hm as measured with a Shore D type hardnessscale of equal to or less than 54; principal component of the basepolymer of said cover being an ionomer resin; said cover having ahardness Hc as measured with a Shore D type hardness scale of equal toor greater than 55; said cover having a weight Wc of equal to or lessthan 5.7 g; and said cover having a ratio (Wc/Hc) of the weight Wc tothe hardness Hc of equal to or less than 0.080.
 2. The golf ballaccording to claim 1 wherein base polymer of said mid layer comprises10% by weight or more and 60% by weight or less styrene block-containingthermoplastic elastomer, and 40% by weight or more and 90% by weight orless ionomer resin.
 3. The golf ball according to claim 1 wherein saidcover has a thickness Tc of equal to or less than 1.2 mm.
 4. The golfball according to claim 1 wherein said mid layer has a thickness Tm of0.5 mm or greater and 1.6 mm or less.
 5. The golf ball according toclaim 1 wherein said reinforcing layer comprises a water-based adhesive.6. The golf ball according to claim 5 wherein base polymer of saidwater-based adhesive is a reaction product between carboxylgroup-containing polyurethane and polycarbodiimide.
 7. The golf ballaccording to claim 1 wherein said reinforcing layer has a thickness of0.003 mm or greater and 0.050 mm or less.
 8. A golf ball which comprisesa spherical core, a mid layer positioned outside of the core, areinforcing layer positioned outside of the mid layer, and a coverpositioned outside of the reinforcing layer, said mid layer having athickness Tm of 0.5 mm or greater and 1.7 mm or less; principalcomponent of the base polymer of said cover being an ionomer resin; saidcover having a hardness Hc as measured with a Shore D type hardnessscale of 56 or greater and 65 or less; and said cover having a thicknessTc of equal to or less than 1.0 mm.
 9. The golf ball according to claim8 wherein said mid layer has a hardness Hm as measured with a Shore Dtype hardness scale of 30 or greater and 54 or less.
 10. The golf ballaccording to claim 1 wherein base polymer of said mid layer comprises30% by weight or more and 60% by weight or less styrene block-containingthermoplastic elastomer, and 40% by weight or more and 70% by weight orless ionomer resin.
 11. The golf ball according to claim 1 wherein basepolymer of said reinforcing layer is a reaction product between carboxylgroup-containing polyurethane and polycarbodiimide.
 12. The golf ballaccording to claim 11 wherein said carboxyl group-containingpolyurethane and said polycarbodiimide are water-based.